1. Field of the Invention
The present invention relates to a buffer circuit applied with a pulse width modulation (PWM) signal and, more particularly, to a PWM buffer circuit for adjusting a frequency and a duty cycle of a PWM signal.
2. Description of the Related Art
In recent years, a fan for dissipating heat generally operates with a fan motor whose speed is dominantly controlled through utilizing a PWM signal. FIG. 1 is a circuit block diagram showing a control circuit for speed of a fan motor by using a conventional PWM control method. Referring to FIG. 1, a PWM signal generation unit 10 outputs a PWM signal S1 to a driving circuit 11. Based on the PWM signal S1, the driving circuit 11 outputs a driving signal A to a fan motor 12, thereby controlling the speed of the fan motor 12. More specifically, one of signal characteristics of the PWM signal S1 is known as “duty cycle,” i.e., a ratio of a pulse width to a period of the PWM signal S1. Assume that the duty cycle of the PWM signal S1 in FIG. 1 is denoted by a reference numeral D1. In the above-mentioned conventional PWM control method, when the duty cycle D1 of the PWM signal S1 is relatively large, the driving signal A output from the driving circuit 11 causes the fan motor 12 to operate at a relatively high speed. On the other hand, when the duty cycle D1 of the PWM signal S1 is relatively small, the driving signal A output from the driving circuit 11 causes the fan motor 12 to operate at a relatively low speed.
However, the conventional PWM control method has at least two disadvantages. The first one of the disadvantages is that the PWM signal S1 to be utilized must have a relatively high frequency, such as 10 kHz or more. When the PWM signal S1 has a frequency lower than 10 kHz, the operation of the fan motor 12 is adversely influenced by switching noise. The second disadvantage is that the duty cycle D1 of the PWM signal S1 to be utilized must be restricted within a range between 30% and 85%, thereby ensuring that the driving circuit 11 and the fan motor 12 can be appropriately controlled by the PWM signal S1.